1. Field of the Invention
The present invention generally relates to high electron mobility transistors (HEMT) and, more particularly, to InP based HEMT that have an improved channel for increased performance.
2. Description of Related Art
High electron mobility transistors (HEMT) are used for microwave and millimeter low noise amplifiers, as well as other RF applications. Those components have been used for various environments such as radar, satellites, and cellular telephones. In millimeter wave communications, the focus has been on GaAs pseudomorphic HEMT and InP HEMT.
Most of the interest in the above two HEMT has been on GaAs. For example, Bolognesi et al., "High-Transconductance Delta-Doped InAs/AlSb HFET's with Ultrathin Silicon-Doped InAs Quantum Well Donor Layer," IEEE Electron Device Letters, Vol. 19, No. 3, pp. 83-85, March 1998 describe an InAs channel layer sandwiched between two AlSb spacer layers. They point out that silicon, which is the usual donor in molecular beam epitaxy (MBE), displays amphoteric behavior in antimonide compounds. Therefore, chalcogenide compounds have been developed for doping, but which have their own disadvantages, such as DX-centers and residual contamination. In an attempt to avoid the disadvantages of chalcogenide compounds, Bolognesi et al. utilized an ultrathin silicon doped InAs quantum well. They believed that doing so produced high transconductance and good channel pinch-off characteristics.
Notwithstanding the fact that GaAs has been the focus of past interest, there has been increasing interest in InP. For an InP substrate, an InGaAs channel has historically always been used. The InGaAs channel has been perceived as having excellent transport properties, providing significant current drive, and having a sufficiently large conduction band discontinuity with InAlAs (the common Schottky material). In growing the InGaAs channel, it has usually been lattice matched to the substrate and made indium rich to increase the conduction band discontinuity and improve transport properties. But doing so has been at the expense of degrading breakdown performance.
An example of an InP substrate with an InGaAs channel is found in Nguyen et al. "Millimeter Wave InP HEMT Technology: Performance and Applications," Solid-State Electronics, Vol. 38, No. 9, pp 1575-1579, 1995. They describe their InP HEMT with particular reference to gain and noise characteristics. Among other things, Nguyen et al. found that their InP HEMT exhibited lower noise and higher gain in comparison to a GaAs pseudomorphic HEMT. However, their InP HEMT exhibited a lower breakdown voltage.
In another example of an InP HEMT with a InGaAs channel, Matloubian et al., "20-GHz High-Efficiency AlInAs--GaInAs on InP Power HEMT," IEEE Microwave and Guided Wave Letters, Vol. 3, No. 5, pp142-144, May 1993 describe their channel layer as being sandwiched between two spacer layers of AlInAs. They indicate that the large conduction band discontinuity, high channel mobility, and high peak velocity in GaInAs result in high transconductance, low parasitic resistances, and excellent high-frequency performance. At 20-GHz, Matloubian et al. believed that their combination of output power, power density, gain and efficiency was comparable to the best results reported for InGaAs on GaAs HEMT.
It can be seen that performance comparisons have focused on a variety of characteristics. Some of them have included breakdown field, energy bandgap, low field mobility, saturated velocity, peak velocity, electric field and thermal conductivity. However, increased performance in one characteristic sometimes leads to a reduction in another characteristic. And even with its advantages, some of the specific disadvantages of the InGaAs channel include the fact that it is narrow in energy bandgap which, in turn, creates a rather low three terminal breakdown voltage. It is also affected by impact ionization and Auger recombination processes. Hence, the building of RF power devices using an InGaAs channel HEMT is restrictive.
Accordingly, there is a need for an improved InP HEMT for RF applications. There is also a need for a channel layer in an InP HEMT that leads to increased performance. The increased performance characteristics that are needed for an InP HEMT include breakdown field, energy bandgap, saturated velocity, peak velocity, electric field and thermal conductivity. Also needed is an improved InP HEMT that can achieve increased performance while also allowing room for adaptability.